When a person turns to Jesus Christ in repentance and faith, he is saved. Jesus becomes his Lord and Savior and he is from that time forward forever in Christ. He becomes a part of the people of God, the body of Christ, the temple of the Holy Spirit. He becomes a new creation.
At the moment of salvation, the Christian also becomes part of the church. After all, the church is simply the one Christ-centered community - the assembly of all believers of all time.
This means that church membership (if that's what you want to call it) is instantaneous. It happens at the same time that salvation does. This is church membership according to scripture.
Any other form of church membership - like the types so often emphasized by local churches today - are little more than artificial man-made dividers that say "who is in and who is out." These have no biblical basis.
So let's all rejoice that when a person comes to Christ he also comes to the church. These happen at the same time. May we recognize this and glorify God for it.